368 PowerPlant Chemistry 2010, 12(6)

PPChem

INTRODUCTION

In Lesson 3 of this course – Underdeposit Corrosion – AGeneral Introduction – presented in the December 2009issue of the PowerPlant Chemistry journal, a generalreview of features common to all underdeposit corrosionmechanisms relevant to boiler and HRSG tubes was given[1]. The following lesson (Lesson 4), which appeared in theFebruary 2010 issue, focused on hydrogen damage,which is the most commonly occurring underdeposit cor-rosion failure mechanism [2], and Lesson 5 (March 2010)dealt with caustic gouging, the second most importantunderdeposit corrosion mechanism [3]. In this lesson, acidphosphate corrosion will be addressed.

LOCATIONS OF FAILURES FOR FOSSIL PLANTSAND HRSGs

Acid phosphate corrosion can only develop in locationswhere excessive deposits, mostly of feedwater corrosionproducts, are formed. In Lesson 4, the locations whereexcessive deposits form in both conventional boilers andHRSGs were discussed [2]. All these locations are thesame for acid phosphate corrosion.

The corrosion products typically originate in the pre-boilerpart of the cycle. They are generated in condensate- andfeedwater-touched cycle components and transportedwith the feedwater into the boiler. Single-phase and two-phase flow-accelerated corrosion of cycle components isthe major source of the corrosion products introduced intothe boiler or heat recovery steam generator. The transportof iron oxides (magnetite or hematite) depends on thefeedwater treatment applied. If copper alloys areemployed in the feedwater system, then copper oxideswill transport into the boiler leaving pure copper within thedeposits.

Acid phosphate corrosion occurs in units experiencingphosphate hideout problems and thus the mechanism ismost often active in the high pressure boilers and HPHRSG evaporators operating above about 10.3 MPa

(1 500 psi). The most susceptible locations are thosewhere both deposition and concentration of boiler watertreatment chemicals (phosphates) occur.

TYPICAL CHARACTERISTICS OF DAMAGE

The most important features of acid phosphate corrosionare summarized in Table 1.

MECHANISM OF ACID PHOSPHATE CORROSION

Damage by acid phosphate corrosion begins with theaccumulation of feedwater corrosion products. The com-bination of the deposits, the local environment with lowsodium-to-phosphate molar ratios (lower than 3:1), andthermal-hydraulic concentration processes leads finally todissolution (fluxing) of the protective magnetite layer onthe boiler or HRSG tube. Sodium phosphates exhibit ret-rograde solubility, i.e., solubility decreases with increasingtemperature. This also assists the local concentrationprocesses.

Phosphate hideout itself does not necessarily lead to acidphosphate corrosion. However, it is important to note thatboth the precipitation (phosphate hideout) and the hydrol-ysis (phosphate hideout return) are incongruent reactionsin which the composition of reaction products differs fromthat of reactants. In both cases solid reaction products aremore acidic (lower sodium-to-phosphate molar ratios) andthe dissolved ones more alkaline (higher sodium-to-phos-phate molar ratios) [4]. When hideout occurs with phos-phate treatments with sodium-to-phosphate molar ratiosless than 3 (such as with congruent phosphate treatment)then this is where the acidic hideout compounds interactwith the magnetite and in some cases with the actualmetal surface: These interactions may occur very rapidly.

Sodium phosphate hideout in high pressure boilers iscaused by reversible reactions between aqueous phos-phate and magnetite that result in the formation of thesodium iron phosphate compounds maricite (NaFePO4)and an iron(III) phase, sodium iron hydroxy phosphate

PPChem 101 – Boiler and HRSG Tube Failures

BOILER AND HRSG TUBE FAILURESPPChem 11 00 11

Acid Phosphate Corrosion

R. Barry Dooley and Albert Bursik

LESSON 6:

Heft2010-06:Innenseiten 21.06.10 17:29 Seite 368

369PowerPlant Chemistry 2010, 12(6)

PPChemPPChem 101 – Boiler and HRSG Tube Failures

(Na4Fe(OH)(PO4)2 · 1/3NaOH) abbreviated as SIHP. Mari -cite is always found within the deposits when acid phos-phate corrosion has occurred and is the key indicator ofthe acid phosphate corrosion mechanism. The iron(III)phase SIHP is unstable under ambient conditions in thepresence of water, and redissolves in boiler water on cool-ing [5].

Hideout from boiler water at low sodium-to-phosphatemolar ratios (≤ 2.0) causes extensive attack on magnetiteto form maricite (NaFePO4) and hematite (Fe2O3), alongwith the formation of neutral, oxidizing conditions in thelocal aqueous phase:

Fe3O4 + 3Na+ + 3H2PO4

– + H2 �� 3NaFePO4 + 4H2O (1)

It is the consumption of hydrogen by Reaction (1) thatdrives the conversion of magnetite to hematite:

Fe3O4 + 2Fe3O4 + H2O �� 3Fe2O3 + H2 (2)

This is probably the mechanism for the severe corrosion ofcarbon steel boiler and HRSG tubes in some stationsoperated under congruent phosphate chemistry treatment[5–8].

At higher sodium-to-phosphate molar ratios (> 2.8), theformation of sodium iron hydroxy phosphate (SIHP) andhydrogen from the oxidation of magnetite by sodium

phosphate may be the major reaction controlling waterchemistry under hideout conditions.

2Fe3O4 + 26Na+ + 12HPO4

2– + 2OH– ��6Na3Fe(PO4)2 · (Na4/3H2/3O) + 4H2O + H2 (3)

The reaction yields local conditions which are reducingand alkaline and thus does not result in serious corrosionof the boiler or HRSG tube as acid phosphate corrosiondoes. The reaction may yield beneficial effects whichinhibit corrosion and thus compensate for modest reduc-tions in pressure tube thickness if the hideout reactiontakes place on the passivating film rather than on loosemagnetite deposits [8].

POSSIBLE ROOT CAUSES

Acid phosphate corrosion requires both the formation ofdeposits and the concentration of acidic phosphates.

Excessive deposits are typically the result of or areencouraged by:

– Poor feedwater treatmenttypically resulting in high corrosion product levels (ironand copper oxides or hydrated oxides). Corrosionproducts generated by corrosion or flow-acceleratedcorrosion in the condensate/feedwater train in conven-tional units, and in the feedwater and in the low pres-

Features of failure• Gouged areas; thick, adherent deposits.• Ductile, thin-edged or pinhole failure.

Effect on internal oxide andcharacteristic deposits

• Low pH at base of deposits leads to dissolution (fluxing) of protective oxide layer.• Two or three distinct layers, of which the inner layer is NaFePO4 (maricite).

Key microstructural features• Similar to caustic gouging (no intergranular hydrogen fissures). The innermost depositlayer is composed of maricite.

• No protective magnetite layer.

Root cause• Heavy deposits caused by a number of processes and too low pH over hideout returnperiods. This is typical for the use of phosphate treatments with sodium-to-phosphatemolar ratios less than 3, such as congruent phosphate treatment.

Cycle chemistry implications

• The local (underneath deposits) sodium-to-phosphate molar ratio is markedly lowerthan 3, in particular during dissolution of hidden phosphates (hideout return).

• During hideout, the bulk sodium-to-phosphate molar ratio increases; in con trast,this ratio decreases underneath deposits. Both an increased blowdown (loss ofsodium!) and dosing of acid phosphates to reduce the bulk sodium-to-phosphate molar ratio exacerbate the local problem by forming an acidicenvironment.

Attack rate • Rapid.

Table 1:

Characteristics of acid phosphate corrosion.

Heft2010-06:Innenseiten 21.06.10 17:29 Seite 369

370 PowerPlant Chemistry 2010, 12(6)

PPChem 101 – Boiler and HRSG Tube FailuresPPChem

sure parts of the HSRG, subsequently deposit in water-walls (conventional boilers) and in the HP evaporatortubing (HRSGs). These heavy deposits are the locatorsof the under-deposit corrosion mechanisms and thus ofacid phosphate corrosion.

Monitoring of total iron around the cycle may supplyinformation on whether the feedwater treatment appliedis optimum or not. In conventional units, the total ironconcentration of < 2 µg · L–1 (all-volatile treatmentreducing) or around 1 µg · L–1 (all-volatile treatment oxi-dizing) or around 0.5 µg · L–1 (oxygenated treatment) isachievable. In combined cycles with HRSGs, operatingwithin the "Rule of 2 and 5" (< 2 µg · L–1 iron in the feed-water and < 5 µg · L–1 in each of the drums) providessome indication of minimum risk for both FAC andunder-deposit corrosion [9].

– Flow disruptions to the internal water flow inside the boiler waterwall orHRSG HP evaporator tubing contributing to increaseddeposition of corrosion products

– Adverse fireside conditionssuch as high heat flux locations, flame impingementand burner misalignment promoting the depositionprocesses

– Depositswhich are not duly detected (disregarding tube sam-pling) and not removed by chemical cleans in a timelymanner

– Ineffective chemical cleanswith deposits remaining on critical places

The cause of the concentration of phosphates underneathdeposits is very often phosphate hideout. Under congruentphosphate treatment chemistry and/or when disodiumand/or monosodium phosphates are added to the boiler orHRSG HP evaporator, the phosphates which hideout havea lower sodium-to-phosphate molar ratio than the bulkboiler water, dissolution of hidden out phosphates (duringshutdown or reduction in load) results in pH decrease bothin the local environment underneath deposits and in theboiler water. The pH reduction in the endangered locationsmay intensify acid phosphate corrosion.

Poor or inadequate instrumentation not meeting the inter-national standard for cycle chemistry instrumentationaccording to the IAPWS Technical Guidance Document[10], which prevents having inadequate cycle chemistrymonitoring and control, typically contributes to acid phos-phate corrosion-related problems.

FEATURES OF FAILURES

Figures 1–4 show examples of acid phosphate corrosionon conventional boiler tubes.

Figure 1:

Hot side of a boiler tube with a gouge caused by acidphosphate corro sion.

Figure 2:

Hot side of a tube where acid phosphate corrosion is active buthas not yet caused a leak as shown in Figure 1.

Figure 3:

Tube with acid phosphate corrosion gouge.

Figure 4:

Acid phosphate corrosion gouges in cross section.

Heft2010-06:Innenseiten 21.06.10 17:29 Seite 370

371PowerPlant Chemistry 2010, 12(6)

PPChemPPChem 101 – Boiler and HRSG Tube Failures

POSSIBLE SOLUTIONS

The possible solutions depend on the extent of damage.For this reason, the condition of the waterwalls or of theHP evaporator tubes of a HRSG has to be evaluated. Acidphosphate corrosion is manifested by tube thinning; ultra-sonic testing is a reliable nondestructive evaluation tech-nique to determine the extent of damage in affected tubesin conventional boilers.

Tube sampling in critical boiler or HRSG regions providesinformation about the type, extent and thickness ofdeposits [9].

Immediate Actions

– Immediately stop using monosodium and/or diso-dium phosphates or mixtures of these for boilerwater treatment. Use either trisodium phosphate plusup to 1 mg · L–1 sodium hydroxide (NaOH) or trisodiumphosphate alone always ensuring that the sodium-to-phosphate molar ratios are greater than 3.

– Establish and apply respective treatment controlranges.

– Take care that a minimum boiler water pH (corrected forammonia) of 9.0 is maintained to provide a reasonablelevel of boiler corrosion protection in the event that acidforming contaminant ingress is experienced. This isparticularly important when low phosphate levels areused. Note that a minimum phosphate concentration of0.2 mg · L–1 or 0.3 mg · L–1 in seawater cooled unitsshould be detectable. To counteract possible ingress ofacidic contamination, some free sodium hydroxidemust be present to ensure that the boiler water pH is≥ 9.0 [7].

If evaluation of tube thinning has indicated dangeroustube conditions, identification of locations and replace-ment of all affected tubes to prevent possible ductile fail-ures is necessary. Wall thinning should never be locallyrepaired by pad welding or canoe/window welds becausethis may increase deposition on the internal surfaces inthe region of the repairs.

Long-Term Actions

Any long-term actions to prevent acid phosphate corro-sion have to focus on optimizing feedwater treatment,minimizing deposit buildup and optimizing boiler waterchemistry. In the event of excessive deposits confirmed bynondestructive evaluation or tube sampling, removal ofdeposits by way of chemical clean might be necessary.

Measures to be taken to minimize deposit buildup are thesame as those recommended in the case of hydrogendamage and caustic gouging:

– Application of an optimum feedwater treatment toensure minimum corrosion product formation andtransport into the boiler.Focus is on the feedwater in conventional plants and onthe feedwater and lower pressure circuits for HRSGs.

– Upgrade cycle chemistry instrumentation to the IAPWSfundamental level and installation of appropriate controlroom alarms.

– Keeping deposits at an acceptable leveland – if necessary – removal of deposits by way ofchemical cleaning.

– Removal of all geometrical flow disrupterssuch as pad welds, backing rings, etc.

– Periodic fireside inspectionsto avoid flame impingement. A proper burner alignmenthelps in reducing flame impingement and excessiveheat flux at critical locations.

Optimization of phosphate treatment is as important asthe measures for minimizing deposit buildup. To preventthe concentration of acidic phosphates underneathdeposits, the following actions are advised:

– Use trisodium phosphate plus up to 1 mg · L–1 sodiumhydroxide (NaOH). The only chemicals under the phos-phate regime which should be dosed in the boiler waterare trisodium phosphate and sodium hydroxide. Thiswill ensure that acid phosphate corrosion will not occureven in cases of severe hideout. [7].

– Alternatively, use sodium phosphate with a sodium-to-phosphate molar ratio of 3:1. This means that the onlychemical dosed in the boiler water should be trisodiumphosphate. If the boiler suffers from phosphate hideoutresulting in an increased sodium-to-phosphate molarratio in the boiler water, on no account should chemi-cals with a lower sodium-to-phosphate molar ratio(disodium and/or monosodium phosphate or mix-tures thereof) be applied. In doing this, the bulk chem-istry will appear to be optimized; however, the sodium-to-phosphate molar ratio underneath deposits will beeven lower than before. In this way, the optimum condi-tions for acid phosphate corrosion will be created.

– Ensure that a minimum boiler water pH (corrected forammonia) of 9.0 is maintained to provide a reasonablelevel of boiler corrosion protection in the event that acidforming contaminant ingress occurs; with low trisodiumphosphate dosing rates some free sodium hydroxidemust be present to maintain the pH at or above 9.0 [11].Minimum phosphate concentration of 0.2 mg · L–1 or0.3 mg · L–1 in seawater cooled units should be alwaysensured.

– Use reliable instruments for boiler water chemistrymonitoringto ensure that any deviations from the normal sodium

Heft2010-06:Innenseiten 21.06.10 17:29 Seite 371

372 PowerPlant Chemistry 2010, 12(6)

PPChem PPChem 101 – Boiler and HRSG Tube Failures / Imprint

and phosphate levels and the pH are detected in suffi-cient time to immediately take counteractive measures.

– Prevent upsets in makeup water systems and conden-sate polishersAll monitoring and alarm systems in these plants haveto be checked for reliability at regular intervals.

REFERENCES

[1] Dooley, R. B., Bursik, A., PowerPlant Chemistry2009, 11(12), 760.

[2] Dooley, R. B., Bursik, A., PowerPlant Chemistry2010, 12(2), 122.

[3] Dooley, R. B., Bursik, A., PowerPlant Chemistry2010, 12(3), 188.

[4] Stodola, J., PowerPlant Chemistry 2003, 5(2), 75.

[5] Tremaine, P. R., Gray, L., Stodola, J., SodiumPhosphate Chemistry under High Pressure UtilityDrum Boiler Conditions, 1992. Canadian ElectricalAssociation, Report 913 G 730.

[6] Tremaine, P. R., Gray, L. G. S., Wiwchar, B., Taylor, P.,Stodola, J., Proc., International Water Con ference,1993 (Pittsburgh, PA, U.S.A.). Engineers' Society ofWestern Pennsylvania, Pittsburgh, PA, U.S.A., 54,186.

[7] Dooley, R. B., Paterson, S., Proc., International WaterConference, 1994 (Pittsburgh, PA, U.S.A.). Engi -neers' Society of Western Pennsylvania, Pittsburgh,PA, U.S.A., 55, 420.

[8] Sodium Phosphate Hideout Mechanisms, 1999.Electric Power Research Institute, Palo Alto, CA,U.S.A., TR-112137.

[9] Dooley, R. B., Weiss, W., PowerPlant Chemistry2010, 12(4), 196.

[10] Instrumentation for Monitoring and Control of CycleChemistry for the Steam-Water Circuits of Fossil-Fired and Combined Cycle Power Plants, 2009. TheInternational Association for the Properties of Waterand Steam.– Downloadable at http://www.iapws.org– PowerPlant Chemistry 2009, 11(10), 606.

[11] Dooley, B., Shields, K., PowerPlant Chemistry 2004,6(3), 153.

ACKNOWLEDGMENT

Wendy Weiss of the Structural Integrity MetallurgicalLaboratory in Austin, TX, U.S.A. supplied all the photo-graphs.

PowerPlant Chemistry® Journal (ISSN 1438-5325)

Publisher: Waesseri GmbHP.O. Box 4338340 HinwilSwitzerland

Phone: +41-44-9402300

E-mail: info@waesseri.com

International Advisory Board:

R. B. Dooley(Structural Integrity Associates, USA)

M. Gruszkiewicz (ORNL, USA)

Professor D. D. Macdonald(Pennsylvania State Univer sity, USA)

M. Sadler (United Kingdom)

U. Staudt (VGB PowerTech, Germany)

R. Svoboda (ALSTOM (Switzerland) AG,Switzerland)

Editor-in-Chief:Albert Bursik, Germany (editor@ppchem.net)

Copyediting and Proofreading: Kirsten Brock, USA/Germany

Graphics and Layout:te.gra – Büro für Technische Grafik, Germany

Production:Appenzeller Druckerei, Switzerland

Frequency:PowerPlant Chemistry® is pub lished monthly

Subscription rates:http://www.ppchem.net/subscription/subscriptionrates2010waesseri.pdf.

International copyright laws protect the journal and all ar ticles. All rights including translation into other languagesare reserved.

The journal may not be reproduced in whole or in part byany means (photocopy, electronic means, etc.) without thewritten permission of Waesseri GmbH.

The authors, the editors, and the publisher do not war rantthe information contained in the journal.

All materials sent to Waesseri may be sub ject to editing.Materials will not be returned if not accompanied by a self-addressed stamped envelope.

Imprint

Heft2010-06:Innenseiten 21.06.10 17:29 Seite 372